Cell Bio 3

Question 1

The cytoskeleton is composed of

Answer 1

¥ Actin filaments
¥ Microtubules
¥ Intermediate filaments

Question 2

Actin polymerizes to form

Answer 2

actin filaments (microfilaments): are flexible fibers 7 nm in diameter and several μm in length, organized into structures such as bundles and 3-D networks

• Very abundant, 5-10% of total protein in eukaryotic cells
• Mammals have 6 actin genes, 4 expressed in muscle cells, 2 in non-muscle cells
• All actins are highly conserved and similar
• Yeast actin is 90% identical to mammal actin
• actin monomers are oriented in the same direction, so actin filaments have polarity; important in their assembly and in establishing the direction of myosin movement relative to actin

Question 3

G actin

Answer 3

actin monomer, has tight binding sites that mediate head-to-tail interactions with two other actin monomers, to form filaments (filamentous [F] actin).

Question 4

Treadmilling

Answer 4

the barbed end of a filament grows 5–10 times faster than the pointed end.
Actin bound to ATP associates with the barbed ends, and the ATP is then hydrolyzed to ADP.
ADP-actin is less tightly bound than ATP-actin, and dissociates at the pointed end, lose here.
Grows usually at + end and loses usually at -

Question 5

Actin-binding proteins

Answer 5

• regulate assembly and disassembly of actin filaments, cross-linking into bundles and networks, and associations with other cell structures

Question 6

Formins

Answer 6

bind ATP-actin and nucleate initial polymerization of long unbranched actin filaments.

Question 7

Profilin

Answer 7

binds actin monomers and stimulates exchange of bound ADP for ATP, increasing the local concentration of ATP-actin.

Question 8

Arp2/3 complex

Answer 8

(actin-related proteins): initiate growth of branched actin filaments, important in driving cell movement at the plasma membrane

Question 9

Tropomyosins

Answer 9

stabilize actin filaments by binding lengthwise along the groove of the filament.
Capping proteins stabilize actin by binding to the barbed or pointed ends.

Question 10

cofilin

Answer 10

severs filaments, making new ends for polymerization or depolymerization

Question 11

Actin filaments are organized into:

Answer 11

Actin bundles—filaments are cross-linked into parallel arrays.
Actin networks—filaments are cross-linked in arrays that form 3-D meshworks with the properties of semisolid gels.

Question 12

Cross-linking proteins have at least two domains that bind actin called:

Answer 12

Actin-bundling proteins are small, rigid proteins that force filaments to align closely.
Actin-network proteins are large, flexible proteins that cross-link perpendicular filaments

Question 13

Two types of actin bundles:

Answer 13

Parallel bundles—closely spaced filaments aligned in parallel, with the same polarity, with barbed ends adjacent to the plasma membrane.
¥ Fimbrin is a bundling protein first isolated from intestinal microvilli.

Contractile bundles—widely-spaced filaments cross-linked by α-actinin dimers.
Increased spacing between filaments allows myosin to interact with the actin filaments.

Question 14

filamin

Answer 14

actin network protein, form flexible cross-links

- A filamin dimer is a flexible V-shaped molecule w actin-binding domains at end of each arm

Question 15

spectrin

Answer 15

is a member of the calponin family of actin-binding proteins
- a tetramer of two polypeptides, α and β. The ends of the tetramers associate with short actin filaments, resulting in the spectrin-actin network.

Ankyrin links the spectrin-actin network to plasma membrane by binding to spectrin and band 3

Protein 4.1 is another link that binds spectrin-actin junctions and glycophorin.

Question 16

Dystrophin

Answer 16

(a calponin) is a linking protein in muscle cells, which links actin filaments to transmembrane proteins in the plasma membrane, which link to the extracellular matrix, helping maintain cell stability during muscle contraction.

Question 17

Integrins

Answer 17

transmembrane proteins that attach fibroblasts to matrix

Question 18

focal adhesions

Answer 18

• sites of attachment, are also attachment sites for large actin bundles called stress fibers

Question 19

Stress fibers-

Answer 19

large actin bundles attaches to focal adhesions, contractile bundles, cross-linked by α-actinin and stabilized by tropomyosin.
Two other proteins, talin and vinculin are involved in binding stress fibers.

Question 20

Adherens junctions

Answer 20

in sheets of endothelial cells, cell to cell contacts form a continuous adhesion belt around each cell.

Question 21

cadherins

Answer 21

transmembrane proteins that mediate contact and bind to cytoplasmic catenins, anchoring actin filaments to the plasma membrane.

Question 22

Microvilli

Answer 22

fingerlike extensions; abundant on cells involved in absorption.
Microvilli of epithelial cells lining the intestine
- form a layer on the apical surface (brush border) of about 1000 microvilli per cell
- increase the surface area for absorption by 10-20x
- contain parallel bundles of 20 to 30 actin filaments cross-linked by fimbrin and villin.
- actin bundles are attached to the plasma membrane by the calcum-binding protein calmodulin in association with myosin I.

Question 23

3 types of microvilli

Answer 23

Pseudopodia are extensions of moderate width, responsible for phagocytosis and the movement of amoebas
Lamellipodia are broad, sheet-like extensions at the leading edge of fibroblasts
Filopodia are thin projections of the plasma membrane supported by actin bundles

Question 24

Movement of a cell across a surface proceeds in three stages:

Answer 24

1. Extension of the leading edge
2. Attachment of leading edge to the substratum
3. Retraction of the rear of the cell into the cell body
   Extension of the leading edge involves branching and polymerization of actin filaments.
   Inhibition of actin polymerization blocks formation of cell surface protrusions.
   Cells move in response to signals from other cells or the environment.

Question 25

myosin

Answer 25

molecular motor—a protein that converts chemical energy (ATP) to mechanical energy, generating force and movement. THICK FILAMENT
Myosin heads hydrolyze ATP, providing energy to drive filament sliding.
Myosin changes shape during repeated cycles of interaction between myosin heads and actin.
Conformational changes in myosin result in movement of myosin heads along actin filaments.

Question 26

Muscle fibers-

Answer 26

large cells formed by fusion of many cells in development, in skeletal muscles
- cytoplasm consists of myofibrils- bundles of thick myosin filaments and thin actin filaments
o a myofibril is a chain of contractile units called sarcomeres
o sarcomeres give skeletal and cardiac muscle their striated appearance
o sarcomeres go from one Z-disk to another
o The bands correspond to presence or absence of myosin filaments.
o Actin filaments are attached at the barbed ends to the Z disc, which includes the cross-linking protein α-actinin.

Question 27

myosin II

Answer 27

(the type in muscle)- has two heavy chains and two pairs of light chains

• heavy chains have a globular head region and a long α-helical tail
• tails twist around each other in a coiled-coil.

Question 28

Sarcoplasmic Reticulum

Answer 28

releases Ca2+ in response to nerve impulse, triggers contraction
- increased Ca2+ concentration in the cytosol affects two actin filament binding proteins: tropomyosin and troponin
o Tropomyosin binds lengthwise along actin filaments, and is also bound to troponins.
o When Ca2+ is absent, the tropomyosin-troponin complex blocks binding of myosin to actin, so NO CONTRACTION
o Binding of Ca2+ to troponin C shifts the complex, allowing myosin to bind to actin

Question 29

Myosin light-chain kinase

Answer 29

catalyzes contraction that is regulated primarily by phosphorylation of a myosin light chain
MLCK is regulated by the Ca2+-binding protein calmodulin

Question 30

Cytokinesis

Answer 30

division of a cell following mitosis.

A contractile ring of actin and myosin II is assembled by membrane-bound myosin just beneath the plasma membrane. Contraction of the ring pinches the cell in two.

Question 31

Myosin I:

Answer 31

Globular head groups act as molecular motors.
Have different structure than Myosin II in muscle
- short tails bind to other structures.
- Movement of myosin I along an actin filament can transport its attached cargo, eg vesicle
- Transport of vesicles and organelles along actin filaments and the movement of plasma membrane during phagocytosis and pseudopod extension

Question 32

Myosin V:

Answer 32

Two-headed dimer that transports vesicles and other cargo along actin filaments; important in neurons.
¥ provides new membrane components to for extension of cell processes

Question 33

Microtubules

Answer 33

are rigid hollow rods that function in cell movements and determining cell shape
- Made of tubulin, dimers of α-tubulin and β-tubulin
- Dynamic structures that undergo continual assembly and disassembly
- Have polarity (plus and minus ends), which determines direction of movement
- Tubulin dimers polymerize to form microtubules: 13 protofilaments around a hollow core
o Protofilaments are head-to-tail arrays of tubulin dimers arranged in parallel.
- γ-tubulin in the centrosome helps in initiating microtubule assembly

Question 34

Dynamic instability

Answer 34

alternating between cycles of growth and shrinkage; microtubules are stabilized at the minus end and rapid GTP hydrolysis results in this instability
• As long as new GTP-bound tubulin dimers are added more rapidly than GTP is hydrolyzed, a GTP cap remains at the plus end and microtubule growth continues.

Question 35

Shrinkage of microtubules

Answer 35

GTP is hydrolyzed more rapidly than new subunits are added so GDP-bound tubulin at the plus end of the microtubule leads to disassembly

Question 36

Microtubule-associated proteins (MAPs)-

Answer 36

regulate the dynamic behavior of MT, the growth or shrinkage of the + ends, - ends are stabilized by proteins that prevent depolymerization.
Polymerase MAPs accelerate growth by increasing incorporation of GTP-bound tubulin.
Depolymerase MAPs dissociate GTP-tubulin from the plus end, lead to microtubule shrinkage

Question 37

CLASP (Cytoplasmic linker associated protein)

Answer 37

proteins rescue microtubules from catastrophe by stopping disassembly and restarting growth.

Question 38

Centrosome

Answer 38

microtubules extend out from here; during mitosis, they extend outward from duplicated centrosomes to form the mitotic spindle that controls separation and distribution of chromosomes to daughter cells; the centrosome is a microtubule-organizing center
The role of centrosomes is to initiate microtubule growth.

Question 39

γ-tubulin ring complex

Answer 39

ring-shaped structure where γ-tubulinin is associated with other proteins; this complex is thought to bypass the rate-limiting nucleation step, speeding microtubule growth

Question 40

Centrioles

Answer 40

centrosomes have a pair of these, are cylindrical, containing nine triplets of microtubules, oriented perpendicular to each other and surrounded by pericentriolar material- initiates microtubules assembly

• also form basal bodies of cilia and flagella
• not found in plant cells, many unicellular eukaryotes, and most meiotic animal cells

Question 41

Axons

Answer 41

microtubules have plus ends towards the tips; associated with tau.

Question 42

Dendrites

Answer 42

microtubules are oriented in both directions; associated with MAP2.

Question 43

Kinesins

Answer 43

move along microtubules toward the plus end.
- move along microtubules in the plus-end direction; they have N-terminal motor domains
- minus-end-directed kinesins have C-terminal motor domains
- some kinesins act as microtubule depolymerizing enzymes
- motor domains are in the middle of the heavy chain (middle motor kinesins)
Kinesin I has two heavy chains (α-helical regions that form coiled-coils) and two light chains

Question 44

Dyneins

Answer 44

move along microtubules toward the minus end.

• Several types of axonemal dyneins power the beating of cilia.
• Cytoplasmic dynein is extremely large with 2–3 heavy chains and a variable number of light and intermediate chains.
• Motor domains of dynein are less well understood than those of kinesins.

Question 45

Cilia and flagella

Answer 45

microtubule-based projections of the plasma membrane, responsible for movement of many eukaryotic cells.
Cilia beat in a coordinated back-and-forth motion, which either moves the cell through a fluid or moves fluid over the surface of the cell.
Flagella are longer, and have a wavelike pattern of beating.

Question 46

axoneme

Answer 46

consists of microtubules in a “9 + 2” pattern: a central pair surrounded by nine outer doublets. Each doublet is a complete A tubule fused to an incomplete B tubule. Nexin links the tubules, and two arms of dynein are attached to each A tubule.

Question 47

Basal body

Answer 47

similar in structure to a centriole, microtubule minus ends are anchored here, has 9 triplets of microtubules, initiate growth of axonemal microtubules and anchor cilia and flagella to the surface of the cell.

Movement of cilia and flagella results from sliding of outer microtubule doublets relative to one another, powered by motor activity of axonemal dyneins.
Dynein bases bind to A tubules, while the head groups bind to B tubules of adjacent doublets.

Question 48

Four types of microtubules make up the mitotic spindle:

Answer 48

. Kinetochore microtubules attach to the condensed chromosomes at the centromeres, stabilizing them.

2. Chromosomal microtubules connect to chromosome ends via chromokinesin.
3. Interpolar microtubules are not attached to chromosomes but are stabilized by overlapping with each other in the center of cell.
4. Astral microtubules extend outward from the centrosomes with the plus ends anchored in the cell cortex.

Question 49

Anaphase A

Answer 49

chromosomes move toward spindle poles along kinetochore microtubules, driven by kinesins that depolymerize and shorten the tubules.

Question 50

Anaphase B

Answer 50

spindle poles separate. Overlapping interpolar microtubules elongate and slide against one another to push the spindle poles apart. Plus-end-directed kinesins cross-link interpolar microtubules and move them toward the plus end.

Question 51

Intermediate filaments

Answer 51

diameters (10-12 nm) between actin filaments (7 nm) and microtubules (25 nm). Not polarized. Not directly involved in cell movements, but provide mechanical strength and place for localization of cell processes. Not found in yeast, plants, and some insects.

Intermediate filaments are composed of many types of proteins expressed in different cells
Primary role of intermediate filaments is probably to strengthen the cytoskeleton of cells in the tissues of multicellular organisms. In tissues, cells are subjected to a variety of mechanical stresses that do not affect cells in a culture dish.

Question 52

Desmosomes

Answer 52

junctions between adjacent epithelial cells.
¥ Keratin filaments attach to dense protein plaques on the cytoplasmic side.
¥ Attachment is mediated by desmoplakin (a plakin family protein).

Question 53

Hemidesmosomes

Answer 53

junctions between epithelial cells and underlying connective tissue or extracellular matrix
¥ Keratin filaments are linked to integrins by different plakins (plectin).

Question 54

Animal cell plasma membranes have five major phospholipids:

Answer 54

1. Outer leaflet—phosphatidylcholine and sphingomyelin.

2. Inner leaflet—phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol

Question 55

Lipid rafts

Answer 55

small semisolid patches that cholesterol and sphingolipids (sphingomyelin and glycolipids) tend to cluster in

transient structures in which specific proteins can be concentrated to facilitate interactions. They are enriched in GPI-anchored proteins and transmembrane proteins.

Question 56

Glycocalyx

Answer 56

carbohydrate coat formed by the oligosaccharides of glycolipids and glycoproteins

• Protects the cell surface from ionic and mechanical stress
• forms a barrier to invading microorganisms
• Oligosaccharides of the glycocalyx participate in a variety of cell–cell interactions.

Question 57

Glycosylphosphatidylinositol (GPI) anchors

Answer 57

added to the C terminus of some proteins in the ER. These proteins are glycosylated and exposed on the cell surface, outside of the cell

Question 58

Transmembrane proteins:

Answer 58

integral proteins that span the lipid bilayer with portions exposed on both sides.

• can be seen in electron micrographs of plasma membranes prepared by freeze-fracture
• membrane-spanning portions are usually α helices of hydrophobic amino acids; they are inserted into the ER membrane during synthesis.
• carbohydrate groups are added in the ER and Golgi; most are glycoproteins with oligosaccharides exposed on the cell surface.

Question 59

Integral membrane proteins

Answer 59

inserted into the lipid bilayer; they can be dissociated only by reagents that disrupt hydrophobic interactions.
Detergents are amphipathic molecules with hydrophobic and hydrophilic groups that can solubilize these proteins.

Question 60

Peripheral membrane proteins

Answer 60

associated with membranes through protein-protein interactions; often ionic bonds. The bonds can be disrupted by polar reagents (salts or extreme pH). Many are part of the cortical cytoskeleton: spectrin, actin, band 4.1, etc.

Question 61

Caveolae

Answer 61

small lipid rafts that start as invaginations of the plasma membrane, organized by caveolin. They have been implicated in endocytosis, cell signaling, regulation of lipid transport, and protection of the plasma membrane against mechanical stress.

Question 62

Aquaporins

Answer 62

allow water molecules to cross the membrane rapidly.

- They are impermeable to charged ions, allowing passage of water without affecting electrochemical gradients.

Question 63

Ligand-gated channels

Answer 63

open in response to binding of neurotransmitters / signaling molecules.

Question 64

Voltage-gated channels

Answer 64

open in response to changes in electric potential across the membrane.

Question 65

Ion pumps

Answer 65

use energy from ATP hydrolysis to actively transport ions across the plasma membrane to maintain concentration gradients so the ionic composition of the cytoplasm is substantially different from that of extracellular fluids.

• Because ions are charged, pumping results in electric gradients across the plasma membrane.
• In resting squid axons, there is an electric potential of about 60 mV; the inside of the cell is negative with respect to the outside.

Na+ is pumped out of the cell while K+ is pumped in.
- The plasma membrane also contains open K+ channels, so the flow of K+ makes the largest contribution to resting membrane potential.

Na higher outside cell
K higher inside cell
2 K for every 3 Na

Question 66

Nerve signaling

Answer 66

As nerve impulses (action potentials) travel along axons, the membrane depolarizes.
Membrane potential goes from –60 mV to +30 mV in less than a millisecond.
Reason why: rapid sequential opening and closing of voltage-gated Na+ and K+ channels.

Depolarization of adjacent regions of the plasma membrane allows action potentials to travel the length of a nerve cell.

At the nerve end, neurotransmitters are released into the synapse where they bind to receptors on another nerve cell to open ligand-gated ion channels.

Question 67

Ligand-gated channels in muscle cells:

Answer 67

Nicotinic acetylcholine receptors
Binding of acetylcholine opens a channel that allows rapid influx of Na+ and K + out of the cell, which depolarizes the cell membrane and triggers an action potential. The action potential then opens voltage-gated Ca2+ channels, increasing intracellular Ca 2+ that signals contraction.

Question 68

Excitation-Contraction Coupling

Answer 68

• Activation of a skeletal myofiber begins with depolarization of the muscle cell membrane or sarcolemma, usually triggered by acetylcholine release at neuromuscular synapses.
• The action potential travels along the plasma membrane into deep invaginations, called transverse tubules, that follow the edges of Z disks.
• The arrival of the action potential in the transverse tubules stimulates the opening of voltage-gated Ca2+ in the SR membrane and the cytosolic Ca2+ concentration in the myofibrils rises.
• Elevated Ca2+ conc. induces a change in two accessory proteins, tropomyosin and troponin C, which are bound to the actin thin filaments and normally block myosin binding. The change in position of these proteins on the actin thin filaments in turn permits the myosin-actin interactions and hence contraction (thin filament regulation)

Question 69

Na+-K+ pump (Na+-K+ ATPase)

Answer 69

uses energy from ATP hydrolysis to transport Na+ and K+ against their electrochemical gradients.

• operates by ATP-driven conformational changes.
• 3 Na+ are transported out and 2 K+ are transported in the cell for every ATP used.
• The Na+-K+ pump uses nearly 25% of the ATP in many animal cells.
• The gradients are necessary for propagation of electric signals in nerve and muscle cells, to drive active transport of other molecules, and to maintain osmotic balance and cell volume.

Question 70

Ca2+ pump

Answer 70

powered by ATP hydrolysis.

• Ca2+ is transported out of the cell or into the ER lumen
• So intracellular Ca2+ concentrations are extremely low.
• Transient, localized increases in intracellular Ca2+ are important in cell signaling (as in muscle contraction).

Question 71

ABC transporters

Answer 71

have highly conserved ATP-binding domains or ATP-binding cassettes.
- More than 100 members of this family in both prokaryotic and eukaryotic cells.
- All use energy from ATP hydrolysis to transport molecules in one direction.
- In prokaryotes, they transport nutrient molecules into the cell.
- In both prokaryotic and eukaryotic cells, they transport toxic substances out of the cell.
- Have two ATP-binding domains and two transmembrane domains.
binding site alternates btwn outward and inward, depending on ATP binding and hydrolysis

Question 72

Cystic fibrosis

Answer 72

mucous layer of the lungs abnormally thick, which inhibits the clearance of pathogens, and the bronchi are therefore less resistant to bacterial infections
- defective Cl– transport in epithelial cells
- CF gene encodes a protein (CFTR/cystic fibrosis transmembrane conductance regulator) in the ABC transporter family; expressed in the lungs, pancreas, intestines, and sweat glands
- result of a mutation in CFTR that interferes with proper folding of the protein
Function of CFTR regulated by cAMP, which activates protein kinase A, which phosphorylates CFTR, activates anion transport thru CFTR, require ATP hydrolysis

Question 73

Symport

Answer 73

transport of two molecules in the same direction; e.g. uptake of glucose and Na

Question 74

Uniport

Answer 74

transport of a single molecule; e.g. facilitated diffusion of glucose

Question 75

Antiport

Answer 75

two molecules are transported in opposite directions; e.g.
• Ca2+ is exported from cells by the Ca2+ pump and by a Na+-Ca2+ antiporter that transports Na+ in and Ca2+ out.
• Na+-H+ antiporter transports Na+ into the cell and H+ out, preventing acidification by H+ produced in metabolism.

Question 76

Macropinocytosis

Answer 76

uptake of extracellular fluids in large vesicles.
- Lamellipodia (sheet-like projections of the plasma membrane) curve into open cups, followed by membrane fusion to form a large intracellular vesicle.

Question 77

Clathrin-mediated endocytosis

Answer 77

is a mechanism for selective uptake of specific molecules.
- Mechanisms of cargo selection, vesicle budding, and vesicle fusion are similar to those involved in vesicular transport in the secretory pathway.

Question 78

Clathrin-coated pits

Answer 78

specialized regions where macromolecules bind to cell surface receptors
- pits bud from the membrane with the help of dynamin, to form small clathrin-coated vesicles; these then fuse with early endosomes.

Question 79

Clathrin-independent endocytosis

Answer 79

does not involve specific membrane receptors or coated vesicles. Macropinocytosis and internalization of caveolae are examples. One pathway mediates uptake of GPI-anchored plasma membrane proteins clustered in lipid rafts.

Question 80

Basal laminae

Answer 80

thin layers on which epithelial cells rest. Also surrounds muscle cells, adipose cells, and peripheral nerves.

Question 81

Different types of extracellular matrices and their components

Answer 81

¥ Tendons—high proportion of fibrous proteins.
¥ Cartilage—high level of polysaccharides that form a compression-resistant gel.
¥ Bone—matrix is hardened by calcium phosphate crystals.

Question 82

Collagen

Answer 82

major structure protein, forms triple helices: three poly-peptide chains are wound together. Triple helix domains consist of repeats of the amino acid sequence Gly-X-Y (a glycine every third position)

Glycine is the smallest AA; allows polypeptides to pack closely together.
Proline is frequently found in the X position; stabilize the helices
Hydroxyproline in the Y position; stabilize the helices.
- Hydroxyproline is formed in the ER by modification of proline in collagen polypeptide chains.
- Hydroxyl groups are thought to stabilize the triple helix by forming hydrogen bonds between polypeptide chains.

Question 83

Type I collagen

Answer 83

(fibril forming) is the most abundant type.

• forms collagen fibrils in which the triple helical molecules form regular staggered arrays [two α1(I) chains and one α2(I)]
• Assembly of fibrils occurs outside the cell from soluble precursor procollagens.

Question 84

hydroxylases

Answer 84

responsible for hydroxylating proline and lysine residues in pro-a chains
requires an essential cofactor, ascorbic acid (vitamin C).
Ð Scurvy results from a lack of vitamin C.
Ð Procollagen chains are not hydroxylated sufficiently to form stable triple helices, nor can they form normal fibrils. So, nonhydroxylated procollagen chains are degraded in the cell.
¥ Blood vessels, tendons, and skin become fragile.

Question 85

Osteogenesis imperfecta

Answer 85

one defective a chain of the three in a collagen molecule can disrupt the whole molecule’s triple helical structure and function. A mutation in a single copy (allele) of either the a1(I) gene or the a2(I) gene, which are located on nonsex chromosomes (autosomes), can cause this disorder. Thus, it normally shows autosomal dominant inheritance.
Type 2- worst type, babies die perinatally

Question 86

type IV collagen

Answer 86

Form networks, most basal laminae is this

The Gly-X-Y repeats are interrupted by short nonhelical sequences, making them more flexible.

Question 87

Elastic fiber connective tissue

Answer 87

is common in organs that stretch and return to shape, such as the lungs; made of elastin which is cross-linked into a network that behaves like a rubber band.

Question 88

glycosaminoglycans

Answer 88

GAGs, polysaccharides that form extracellular matrix gels, are repeating units of disaccharides, linked to proteins to form proteoglycans
addition of sulfate groups make GAGs highly negatively charged- bind positively charged ions and trap water molecules to form hydrated gels.

Question 89

Aggrecan

Answer 89

the major proteoglycan of cartilage, has 100 chains of chondroitin sulfate attached to a core protein.

Question 90

Hyaluronan

Answer 90

is the only GAG that is a single long polysaccharide chain, no sulfate groups.
- synthesized at the plasma membrane by a transmembrane hyaluronan synthase.

Question 91

Fibronectin

Answer 91

is the main adhesion protein of connective tissues, which is cross-linked into fibrils.
- It has binding sites for both collagen and GAGs, thus enabling cross linking.